In a speed control device including a speed instruction generating unit, an addition computing unit, a speed control unit, a current controlling unit, an inertia estimating unit, an inertia setting unit, a switching unit and a constant setting unit, when it is possible to perform accelerating or decelerating operation of the electric motor to estimate the inertia moments of the electric motor and a driving target thereof, an operation is carried out to derive an inertia moment estimation value. The proportional gain of the speed control unit is set through the switching unit and the constant setting unit on the basis of the inertia moment estimation value. When it is impossible to perform the above operation, the proportional gain of the speed control unit is set through the switching unit and the constant setting unit on the basis of the inertia moment manual set value.